Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells ("twisted nematic") and STN cells ("super-twisted nematic") having a twisted nematic structure, SBE cells ("super birefringence effect"), phase change cells having a cholesteric-nematic phase transition and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
Further, electro-optical devices based on chiral tilted smectic liquid crystals have been proposed in Appl. Phys. Lett. 36, 899 (1980) and in Recent Developments in Condensed Matter Physics 4 , 309 (1981). In such cases the ferroelectric properties of these materials are used. As the tilted smectic phases there are suitable, for example, smectic C, F, G, H, I and K phases. There are generally preferred smectic C phases which permit especially high response speeds. The chiral tilted phases are usually denoted by S.sub.C * S.sub.F * etc , with the asterisk indicating chirality.
The liquid crystal materials must have a good chemical and thermal stability and a high stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have low viscosity and in the cells should give short response times, low threshold potentials and a high contrast. Furthermore, at the usual operating temperatures they should have a suitable mesophase, for example a nematic, cholesteric or chiral tilted smectic phase. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible. Besides the general interest in liquid crystal materials having a high optical anisotropy, there has recently been an increased interest in materials having a low optical anisotropy, especially for actively addressed liquid crystal indicators, for example, in the case of TFT applications (thin film transistor) in television sets. On the other hand, chiral tilted smectic liquid crystals should have a sufficiently high spontaneous polarization.
In order to optimize the properties, liquid crystals are generally used as mixtures of several components. It is therefore important that the components have a good miscibility with one another. Cholesteric mixtures can preferably consist of one or more optically active dopants and a nematic liquid crystal material and ferroelectric liquid crystals can preferably consist of one or more optically active dopants and a liquid crystal material having a tilted smectic phase.
DE-A-26 17 593 and DE-A-32 25 290 disclose liquid crystal components in which two or more rings can be linked via a bridging group having more than 2 chain atoms, for example via a --(CH.sub.2).sub.4 -- group. Enantiotropic mesophases have, however, only been found for tetracyclic compounds (as well as for certain tricyclic compounds which, however, have an ester group in the bridging group) and the clearing points lie significantly lower than the case of corresponding compounds having 2 chain atoms in the bridging group or with directly linked rings.